This invention relates to the thermoforming of articles from polyethylene terephthalate modified with polyolefins. In particular, the invention describes a thermoforming process for making a partially crystalline, heat set article from high molecular weight polyethylene terephthalate modified with a polyolefin, thereby achieving an improved process exhibiting superior mold release, shorter cycle times and broader operating temperature ranges. The articles produced exhibit improved impact resistance and improved high temperature service life.